Gabriele Morosi

Positronium chemistry by quantum Monte Carlo. I. Positronium-first row atom complexes

The stability of the ground and excited states of Positronium–atom complexes [A, Ps], Ps =[e+,e−], has been explored for A=Li, B, C, O, F using variational and diffusion Monte Carlo techniques. From the numerical results of our simulations it turns out that the ground state of the complexes [Li, Ps]2S, [C, Ps]3S, [O, Ps]1P, and [F, Ps]2S is stable against the break up in the two neutral fragments A and Ps, while the ground state of [B, Ps]2P has an energy above the same dissociation threshold. As to the excited states, the only possible candidate, [F, Ps]2P, has a total energy statistically equal to the lower dissociation threshold, i.e. it does not seem to be stable against the dissociation.

Robust wave function optimization procedures in quantum Monte Carlo methods

The energy variance optimization algorithm over a fixed ensemble of configurations in variational Monte Carlo often encounters problems of convergence. Being formally identical to a problem of fitting data, we re-examine it from a statistical maximum-likelihood point of view. We show that the assumption of an underlying Gaussian distribution of the local energy, implicit in the standard variance minimization scheme, is not theoretically nor practically justified, and frequently generates convergence problems. We propose alternative procedures for optimization of trial wave functions in quantum Monte Carlo and successfully test them by optimizing a trial wave function for the helium trimer.

An investigation of the geometry of the CH+5 ion by the CNDO method

Abstract The geometry of the CH+5 ion has been investigated employing the semiempirical LCAO SCF (CNDO/2) method. A configuration with Cs symmetry has been found to be the most stable.

Quantum Monte Carlo investigation of small 4He clusters with a 3He impurity

Small helium (4He) clusters containing the lighter isotope 3He are studied by means of quantum Monte Carlo methods. Accurate ground state energies and structural properties are obtained using accurate trial wave functions and the Tang–Tonnies–Yiu (TTY) helium–helium pair potential. The dimer 4He–3He is not bound; as well as the trimer 4He3He2. The smallest cluster containing 3He is 4He2 3He with a nonrigid structure having a marked linear contribution. Interestingly, this weakly bound system, with an energy one order of magnitude less than the 4He3 trimer, is able to bind another 3He atom, forming the tetramer 4He2 3He2, which shows the odd feature of having five out of six unbound pairs. In general, the substitution of a single 4He atom in a pure cluster with a 3He atom leads to an energetic destabilization, as the pair 4He–3He is not bound. The isotopic impurity is found to perturb only weakly the distributions of the remaining 4He atoms, which retain the high floppiness already found in the pure cluste...

Positron chemistry by quantum Monte Carlo. II. Ground-state of positron-polar molecule complexes

@LiF,e 1 # 2,1 S 1 is stable against the dissociation either in the two fragments M and e 1 or in the other two fragments M 1 and Ps5@e 1 ,e 2 #, while the ground-state of @H2O,e 1 # 2,1 A1, and of @HF,e 1 # 2,1 S 1 has an energy equal to the dissociation threshold, M and e 1 . We also compare the predicted vertical positron affinity ~PA! with high quality vertical electron affinity ~EA! and discuss the relevant difference between the two values.

Ground state and excitation dynamics in Ag doped helium clusters

We present a quantum Monte Carlo study of the structure and energetics of silver doped helium clusters AgHen for n up to 100. Our simulations show the first solvation shell of the Ag atom to include roughly 20 He atoms, and to possess a structured angular distribution. Moreover, the 2P1/2←2S1/2 and 2P3/2←2S1/2 electronic transitions of the embedded silver impurity have been studied as a function of the number of helium atoms. The computed spectra show a redshift for n⩽15 and an increasing blueshift for larger clusters, a feature attributed to the effect of the second solvation shell of He atoms. For the largest cluster, the computed excitation spectrum is found in excellent agreement with the ones recorded in superfluid He clusters and bulk. No signature of the direct formation of the proposed AgHe2 exciplex is present in the computed spectrum of AgHe100. To explain the absence of the fluorescent D2 line in the experiments, a relaxation mechanism between the 2P3/2 and the 2P1/2 states is proposed on the b...

Positron and positronium chemistry by quantum Monte Carlo. IV. Can this method accurately compute observables beyond energy

Many different properties of the positron containing systems PsH, [Li,e+], LiPs, and [LiH,e+], were computed using both variational Monte Carlo and fixed node diffusion Monte Carlo methods, and explicitly correlated trial wave functions. Our results show that these techniques can accurately compute not only energy values, but also other observables. Our 〈δ(r+−)〉 values for PsH, [Li,e+], and LiPs are in good agreement with the most recent state of the art correlated calculations, while for [LiH,e+] our calculations are the first to give reliable results.

Positron and positronium chemistry by quantum Monte Carlo. V. The ground state potential energy curve of e+LiH

The potential energy curve of e+LiH has been computed by means of diffusion Monte Carlo using explicitly correlated trial wave functions. This curve allows us to compute the adiabatic total and binding energies and the vibrational spectrum of e+LiH, and the adiabatic positron affinity of LiH. Using these results, we discuss the possibility to detect spectroscopically e+LiH in the gas phase, in order to have the first direct observation of a positron-containing system.

Comparison of different propagators in diffusion Monte Carlo simulations of noble gas clusters

Several short-time approximations of the imaginary-time propagator of the Schrodinger equation are compared working on small helium and neon clusters. A recently discussed fourth order short time approximation of the propagator [Phys. Rev. E 61, 2050 (2000)] allows us to compute several properties practically unaffected by the time step bias. The comparison among simulations of the same length shows that this algorithm permits the use of larger time steps, leading to more accurate statistics than the ones obtained by employing commonly used schemes. Results of the mixed estimator of the potential energy, of the first two momenta of the interparticle distribution, and of the particle-center-of-mass distribution seem to indicate that the new propagator is able to perform unbiased sampling even when very large time steps are used. Also, the relative population of the four Ne7 isomers sampled using the fourth order propagator does not show any time step bias in the 200–1000 hartree−1 time step range. This fac...

Positron and positronium chemistry by quantum Monte Carlo. VI. The ground state of LiPs, NaPs, e+Be, and e+Mg

The ground states of the positronic complexes LiPs, NaPs, e+Be, e+Mg, and of the parent ordinary-matter systems have been simulated by means of the all-electron fixed-node diffusion Monte Carlo (DMC) method. Positron affinities and positronium binding energies are computed by direct difference between the DMC energy results. LiPs was recomputed in order to test the possibility of approximating the electron–positron Coulomb potential with a model one that does not diverge for r=0, finding accurate agreement with previous DMC results. As to e+Be, the effect due to the near degeneracy of the 1s22s2 and 1s22p2 configurations in Be is found to be relevant also for the positron affinity, and is discussed on the basis of the change in the ionization potential and the dipole polarizability. The DMC estimate of the positron affinity of Mg, a quantity still under debate, is 0.0168(14) hartree, in close agreement with the value 0.015 612 hartree computed by Mitroy and Ryzhihk [J. Phys. B. 34, 2001 (2001)] using expl...